I'll give you the quick answer by memory and let the next guy correct the details.
All 610s have FL barrels. It appears S&W makes a production run, it takes a while for the market to absorb them, then S&W lets demand build up before doing another run, so it's easy to group them by production run or time.
1989, No-dash, 1st batch, 4,560 guns, 6 1/2" & 5" but mostly 6 1/2", fluted cylinders, wood combat smooth finger groove stocks, forged parts, black pined front sight blade, 48 oz. in 6 1/2"
1998, 2nd batch, dash-1 & dash-2, unfluted cylinders, modern rear sight drilled & tapped for scope underneath, front sights interchangeable with your fingers, rubber Hogues, all 6 1/2", except for a small order of 3" (300?) by Lew Horton, 50 oz. in 6 1/2". Dash-2 changed to MIM
2001, 3rd batch, unflutted cylinders, dash-2, easily changed front sights, 3 7/8", rubber Hogues, 42 oz., incorrectly advertised as 50 oz.
2002, 4th batch, dash 3, lock added, 3 7/8" & 6 1/2", flutted cylinders, rubber Hogues, I think this batch are the only ones that were cataloged.
The SCSW describes dash 2s & 3s as 4" but my dash 2's box is labeled 3 7/8" and that's how it measures.